10:03 a.m.: After learning of the other attacks, passengers on United Airlines Flight 93 launch a counterattack on hijackers aboard their plane to try to seize control of the aircraft. In response, the hijackers crash the plane into an empty field near Shanksville, Pa.

10:28 a.m.: The North Tower (1 WTC) collapses. The 16-acre World Trade Center site is in ruins, with collateral damage affecting all adjacent properties and streets. A rescue and recovery effort begins immediately.

Ceremonies will also be held at the Pentagon and in Shanksville, Pennsylvania.